As is known, the provision of adequate joint systems is extremely important for the installation and the correct operation of pipes made of any material.
For this reason, in addition to research aimed at studying and producing the materials most suitable for manufacturing pipes, an extremely large number of types of joint have been proposed, made and tested.
Regardless of the method by which they are made, joints must comply with some fundamental and fully general requirements.
First of all, they must ensure perfect tightness during their entire period of operation, have a durability equal to the required durability of the pipe and have the same characteristics of resistance to the chemical, thermal and mechanical stresses to which the pipes are subjected.
The joints must, furthermore, be made easily and rapidly and must be economical so as to affect as little as possible the overall cost of the pipe.
For example, in the case of joints for metallic pipes for conveying liquids, such as water, the pipes must be coated with lining substances which withstand chemical and mechanical actions.
These lining substances can normally be, for example, epoxy paints or plastic linings importing to the pipe greater chemical-mechanical resistance and therefore greater durability.
Every time it is necessary to provide a weld for forming a joint between one pipe and another, the high temperature which is produced during welding deteriorates the heat sensitive lining, triggering all the disadvantages which can derive from the loss of the lining.
In order to obviate this disadvantage, ball joints for metallic pipes have already been studied wherein an inner lining is provided in the male part of each pipe and is subsequently welded on three faces to fix it to the inner surface of the pipe.
After welding, the inside of the pipe is cleaned by means of the methods usually employed, and then the lining which is resistant to chemical and mechanical actions is applied.
With this known system, the inner lining of the pipe is spaced from the welding region of the joint between two consecutive pipes so as to prevent the transmission of the heat generated by welding and thus the deterioration of the pipe lining.
However, the above described solution has numerous disadvantages.
For example, it is necessary to perform inside welds on three faces which are not always readily accessible, especially for small-diameter pipes.
The presence of the inside the pipe, welds which cannot be ground, furthermore, results in an irregularity of the pipe, and therefore an increase in the turbulence of the fluid during its passage through these regions.
It should also be noted that the lining in the regions above or adjacent to the welds is seldom perfectly uniform, with a consequent degradation in the quality of the finished product.
Finally, it should also be mentioned that in order to make the welds inside each pipe it is necessary to use specialized personnel and special equipment.
As a consequence, the provision of the prior joint entails considerable manufacturing time and high costs.